This invention relates to circuits for use with gas discharge lamps such as deuterium lamps of the type used as light sources in ultraviolet spectrophotometers.
In one class of known circuit for gas discharge lamps, the cathode is preheated and then a high potential is applied between the cathode and the anode to break down the gas, after which the high potential is removed and a lower sustaining potential at a regulated current is supplied.
In a prior art circuit of this type, the preheat current through the cathode is reduced or discontinued once the lamp is in operation since the cathode is heated by ion bombardment and by the arc current which passes through it during operation of the lamp. The potential applied to the gas discharge during operation is a low potential, which may be a regulated DC or pulsed current, and it is applied across the anode and the cathode. The reduced preheat current may be regulated DC or unregulated AC. Such circuits are described in the publication "Characteristics and Use of Deuterium Lamps" application RES-0793 published in 1980 by Hamamatsu TV Co. Ltd. with offices at 1126 Ichinocho, Hamamatsu, Japan, and 420 South Avenue, Middlesex, N.J., U.S.A.
These prior art circuits have several disadvantages such as: (1) the tubes they control have a short operating life under some circumstances due to evaporation of the oxide coating on the cathode or sputtering of the cathode from ion collision; and (2) the tubes produce, under some circumstances, excessive photometric noise.